In general, active pixels comprise a photodiode and three, four or five MOS transistors making it possible to control the reading of the charge generated by light in the photodiode. Pixels with four transistors operate by firstly transferring from the photodiode to a capacitive storage node the charge generated by light, and then by carrying the potential of the storage node over to a column conductor; one of the transistors serves to reinitialize the potential of the storage node before the transfer of charge from the photodiode to the storage node. Pixels with five transistors furthermore comprise a transistor for reinitializing the potential of the photodiode.
A mode of operation of the sensor with a double duration of integration at each image frame is described in patent publication WO2010/066850. After a first duration Ti1, the charge engendered by the photodiode is transferred into the storage node. Next, slightly before the end of a second duration Ti2, longer than the duration Ti1, the potential of the storage node is sampled in a capacitor of a reading circuit, the potential of the storage node is reinitialized, the reinitialization potential is sampled in another capacitor, and the charge resulting from the second duration of integration is transferred from the photodiode to the storage node. Thereupon, the level of the potential of the storage node is tested. If it indicates a risk of saturation of the pixel, then one will use the samples already stored in order to carry out on the basis of their difference an analog-digital conversion representing the illumination level received by the pixel. If conversely the level of the potential of the storage node shows that there is no risk of saturation, a third sampling is done which replaces in the first capacitor the previously stored level, and this new sample is employed in order to do the analog-digital conversion of the difference between the third sample and the second.
A third conditional sampling is therefore done as a function of the level of the charge gathered during the second duration of integration. The choice is made pixel by pixel at the moment of the reading of a row of pixels.
Provision may also be made for the second duration of integration to be shorter than the first. In this case a test is performed on the level of potential of the storage node before the second transfer to the storage node; this is the potential resulting from the longer integration. This time, if there is a risk of saturation, the third sampling will be done before doing the analog-digital conversion; if there is no risk of saturation it will not be done and the analog-digital conversion will be done on the basis of the first and of the second sample.
In the case where it is the level resulting from the shorter duration of integration which is converted into digital, the result of the conversion is multiplied by the ratio of the longer duration to the shorter duration so as to refer the value to the same scale as the result of the conversion of the charge level arising from the longer integration.
It has been noted that this test for deciding on the third sampling exhibited drawbacks.
Firstly a comparator (bulky) is necessary for each column. Then, the test is done on an absolute value of the potential of a column conductor receiving the level of potential of the storage node at the moment of selecting a row of pixels. This potential is compared with a threshold which defines the risk of saturation. The test therefore pertains to an absolute value of level of potential, not to a differential value (useful level of potential minus reinitialization level). This creates an uncertainty in the choice of the threshold to be used. Moreover, there are other uncertainties in the value of the threshold that must be applied to the comparator in order to detect a risk of saturation. These uncertainties are related to variations in the manufacturing method, to temperature variations, to the differences in offset voltage of the reading transistors in the various pixels, to the differences of offset voltages of the comparators corresponding to the various columns of the matrix of pixels. It is therefore mandatory to take significant margins in order to determine the threshold making it possible to know whether or not there is a risk of saturation. This entails a loss of dynamic range, that is to say a reduction in the interval of illuminations that the sensor can actually measure for a given duration of integration. Finally, the necessity to do this voltage comparison at a moment other than the moment of the sampling of a level of potential of the column conductor entails additional current consumption. Indeed, the column conductor is energized only during the phases where it has to copy the level of potential of the storage node. Here, it must be energized both during the test and during the third sampling which is posterior to this comparison.